To keyword or not to keyword?

Depends on how solid and familiar the key words are. I like learning some subset of the language of a new game. If they are functionally made up words or regular words that have definitions uncommon to other games then 15-20 would be too many to have to rely on a glossary.

Classically, lifeleech is as MayuPolo describes. When you deliver damage, you heal for as much damage as you delivered. In fact the first three are all pretty common terms that I've seen. Having those clarified in the rules makes sense, per the original question. The concept behind enmity seems common enough, though I don't recall a game calling it enmity. But if there are only a couple of those terms that give the game its own flavor, then I think it's cool to just use the keywords on the cards.

I actually think it has less to do with the quantity of keywords, and more to do with the frequency in which they appear. Adding a key word to a card (or ability) that only appears a couple of times to save space on the card is probably not a good idea. I might be better to drop the card, or come up with a succinct wording. If an ability is appearing on, say, a bunch of cards, then its good to have it.

Just something I looked up for a point of reference. Unlimited Edition in magic the gathering had (I think..) 296 cards. The keyword ability "First Strike" appears on 4 of them (one is an enchantment that grants it). "Trample" on 5 of them. "Flying" is referenced on 32 cards. "Banding" is on 4.